1. Field of the Invention
This invention relates to abrasive tools, and more particularly to abrasive inserts adapted for use in grinding the surface of bimetallic engine blocks.
2. Background Information
As automakers push to reduce the weight of automobiles, the engine block remains one of the heaviest single components. Manufacturing the engine blocks in a bimetallic manner, such as by fabricating the blocks from aluminum and placing cast iron sleeves into the cylinder bores can substantially reduce the weight of the engine block relative to conventional cast iron engine blocks. An important aspect of the engine block manufacturing process, however, is to provide the block with a flat or planarized upper surface or fire deck for mating with the cylinder head. Machining of conventional unimetallic engine blocks (i.e. cast iron) is generally accomplished by common machining processes such as fly cutting or high speed milling utilizing hardened ceramic inserts, such as silicon nitride, tungsten carbide or polycrystalline diamond (PCD), on the milling head. This process using PCD inserts has also now been adopted for use in machining bimetallic blocks. Although satisfactory when utilized for unimetallic blocks, this approach tends to produce undesirable results when used with blocks fabricated from two materials, one of which is soft, i.e., aluminum, and the other of which is brittle, i.e., cast iron. When utilized to mill bimetallic parts, the relatively expensive PCD inserts tend to wear rapidly. Moreover, to insure a smooth and flat surface, multiple passes with the milling inserts are typically utilized, although score lines may still be seen. Waviness also sometimes occurs in the surface of the fire deck. These problems may be associated with, or exacerbated by, the differences in optimal milling tool configuration for soft versus brittle materials. For example, most high-speed milling cutters made for softer materials, such as aluminum, operate most efficiently at substantially greater rake angles than those used for harder materials such as cast iron. Clearance angles, or the angle between the land and a tangent to the cutter from the tip of the tooth, also depend on the various work materials. Cast iron typically requires values of 4 to 7 degrees, whereas soft materials such as magnesium, aluminum, and brass are cut efficiently with clearance angles of 10 to 12 degrees. (See, e.g., B. H. Amstead et al. Manufacturing Processes, 1977, pp. 555-556).
One solution to this problem has been to countersink the cast iron sleeves to the depth to which the aluminum is to be removed. Once countersunk, the aluminum block may then be milled in a conventional manner to bring the aluminum to the predetermined height and flatness. While this approach has been used successfully to planarize fire decks of bimetallic engine blocks, the step of countersinking the cast iron sleeves disadvantageously adds an extra machining step, an extra tool change and an extra tool set up which tends to increase the time and expense of engine block fabrication. It is thus desirable to devise a tool and/or process able to planarize the fire deck of a bimetallic engine block in a single pass or process step.
Another technique commonly utilized for metal removal involves use of conventional grinding wheels, typically face grinding wheels or surface grinding wheels comprising alumina grain in resin bond. While this technique tends to be effective on cast iron workpieces, aluminum is relatively soft, gummy and abrasive, and thus difficult to grind. Another drawback of this approach is that these bonded abrasive grinding wheels generally cannot be used in conventional milling machines, due to differences in the parameters associated with milling versus grinding. One such difference is the need for guarding to protect machine operators from debris expelled during use of the bonded abrasive grinding wheel. This need for having discrete grinding machines in addition to conventional milling machines, disadvantageously tends to increase the cost of producing bimetallic engine blocks in this manner, due to increased overhead in terms of capital equipment costs and manufacturing space requirements, etc.
Thus, a need exists for an improved tool and/or method for machining fire decks of bimetallic engine blocks.
A significant reason for the difficulty associated with milling bimetallic workpieces is that during the milling operation, each blade or insert of the milling head is maintained in relatively interrupted contact with the bimetallic block, in which the blade repeatedly takes relatively large cuts across the boundary between the soft aluminum and the brittle cast iron as the milling head rotates. The relatively large number of cutting points provided by each abrasive grain of a grinding wheel provides a more continuous contact with the workpiece, in which each grain takes a relatively smaller cut or bite as it crosses the boundary between materials.